Insects and colour

Mindboggling examples abound of spectacular beauty and creativity, with no obvious evolutionary purpose except perhaps to stupefy an art-loving human beholder. A quick search on Pinterest or Google should convince you.

However, colour usually has more mundane purposes: predator-avoidance, mate-recognition and suchlike. Yet even these everyday functions can take on bizarre and fascinating forms.

But let’s go back to the beginning. What causes colour in the first place?

How do colours form?

Colour in nature is produced in four different ways:

selective absorption of light by pigments,

scattering or refraction of light (structural colours),

fluorescence (where light is absorbed in one wavelength, eg. UV, and re-emitted in a longer wavelength, eg. green) and

by production of light (bioluminescence).

Pigments

The most common source of colour is pigmentation. Coloured pigments absorb certain wavelengths of light and not others. Whatever light waves are not absorbed, bounce off or shine through: now you see white light minus the absorbed wavelengths. This is high school science, as far as I remember.

Some pigments are found only in a single family of insects, such as papiliochromes (yellow, red-brown) are found only in swallowtail butterflies, aphins (red, purple, black) only in aphids, and anthraquinones (red, violet, green, blue) in scale insects. The pigments carmine and crimson, which Michelangelo painted with, are derived from scale insects. Carmine is known nowadays as Natural Red 4, cochineal or E120. Because of its animal origin it is used in food, cosmetics and medicines.

Pigments are often waste-products. Sometimes they have been ingested or derived from ingested pigments, such as carotenoids and flavoids. Others are manufactured by the insect. In combination, various different pigments can create a full, vibrant palette. As you can imagine there is some serious biochemistry going on there, much of it not yet fully understood.

One fascinating fact is that the genes coding for colour are usually involved in other body functions. For example, melanin plays a role in immunity, wound healing and water retention. Pterins can be important cofactors of metabolic enzymes involved in growth. It is therefore possible that certain colourations that we see and try to interpret, actually have no visual purpose at all, or only a secondary purpose. The pigments may have been selected for other reasons, or are simply by-products of more important body functions.

The ‘yellow’ gene is involved in sex-specific behaviour. In many yellowish butterflies the males and females are different levels of yellow, begging the question: do the colour differences have a specific purpose or is it just a by-product of more important gender issues? Or perhaps both?

Structural colour

While pigments involve chemistry, structural colours involve physics. Structural colours are formed by the microscopic (or rather, nanoscopic) structure of the insect skin, that reflects, refracts, diffracts or scatters the light in ways that produce colour.

Light that gets scattered by rough surfaces is usually white. But when the irregularities on the surface are small enough, short wavelengths (blue light) get scattered while longer wavelength just bounce off. This produces a powder or sky blue (‘Tyndall blue’) as in this cuckoo bee.

Iridescence results when light waves interfere with each other. This interference is also caused by nanoscopic irregularities on the surface. Wavelengths that get out of phase cancel each other out, and so disappear from the spectrum. Those that find themselves in phase, enhance each other to greater brilliance. The insect world is ripe with beautiful examples.

Green dung beetle

A cuckoo wasp with spectacular iridescent bands of rainbow colour. The bands must have slightly different nanoscopic surface structures. In what we perceive as yellow bands, blue wavelengths get out of phase, and so ‘erase’ each other, while the yellow wavelengths are in phase and, as they interact, become twice as bright. In the blue bands the opposite happens. Phenomenal!

Structural colours can be used in conjunction with pigments. An iridescent blue can be mixed with red pigment to create a luminous purple. Pigments may also absorb stray rays of the ‘wrong’ colour light to make the iridescent effect more pure and stand out more brilliantly.

One amazing fact about structural colours is that they have been found in 50 million-year-old fossilized insects. Pigments disintegrate, but under certain conditions the nanostructure of the skin can be preserved, and works today as well as it did then. Check it out here or here.

Bioluminescence

Light production through bioluminescence is the forte of fireflies and glow-worms (a family of beetles). The function of firefly/glow-worm light is communication: males and females signal to each other with species-specific light codes. Some predatory females imitate the codes from other species and when hopeful suitors arrive, they eat them up.

A gigantic female glow-worm from Morningside, Durban.

This is a male firefly. His amazing light organ is clearly visible.

The light organ in adults is a highly efficient instrument. It is covered with microscopic scales whose structure transmits light extremely well. When scientists copied this design, they found that LED lights coated with artificial firefly skin shone one and a half times brighter than normal LEDs. See here.

Functions of colour

Mate recognition

Butterflies and many other insects are short-lived. Butterflies apparently lose their interest in the opposite sex within a day or two after emerging, because their pheromones become exhausted. Many insects have even less time. Some mayflies have a minute or two. They only live for five! So, finding a mate quickly is of utmost essence.

Often, the first step in recognising a potential mate is visual. In this area, colour helps a lot. In insects, sexual dimorphism – meaning males and females look different – is quite common. Apart from anatomical or body size differences, the sexes are sometimes coloured differently, aiding recognition and competition.

In these damselflies, the males sport bright colours. The females are dull brownish.

Male yellow pansies like to sit in an open area, displaying their lovely colours. They use clear visual cues to patrol their territory and to attract a mate.

Predator avoidance

Insects are a key component in terrestrial food webs. Which means in short: they get eaten. Unless they can avoid it somehow.

There are several options: you can try run, jump, fly or swim away real quick, you can burrow or otherwise hide away, even inside d.i.y. containers, you can bite, sting, shoot, with or without chemical weapons, and so on. Insects do use brute force. But they also have some sneaky tricks up their sleeves.

Whatever strange and wonderful strategies insects use to avoid being eaten, colouration often plays an important role.

Crypsis (avoiding detection)

Crypsis is about concealment, hiding out in the open. Simple ‘background matching’ can be extremely effective. Brown and green are favourite colours when you are in the camouflage business, and living in nature.

Beaded weevils feed on foliage, but when disturbed they drop to the ground and freeze. In their mottled browns, they are virtually impossible to find again.

This green lacewing, with its transparent wings and black ‘leaf spots’, is difficult to see in green foliage.

Garden locusts as adults are brown with bold patterns. But green is a useful colour when you are little and hungry. Most grasshoppers, whatever colour their adult attire may be, start their lives in shades of green or brown.

Cricket: general mottled brown is a useful colour when you are a helpless and edible, and when you spend a lot of time on the ground.

But crypsis goes further than general ‘blending-in’. Many insects position themselves on purpose where their designer-colours exactly match their surroundings, making them near invisible.

Some go for full disguise: looking completely like a natural object, a stone perhaps or a twig, leaf or flower – in shape, colour, texture and posture. Such ‘advanced’ camouflage is particularly common in mantids, grasshoppers, moths and caterpillars, and of course, stick insects.

Bark mantis

Young flower mantis

Leaf katydid

Grass mimicking grasshopper

Stone mimicking groundhopper

Looper caterpillar

Gamboge thorn moth

Green hawk moth: this moth (probably a kind of hawk moth) matches the green and brown bark of a tree.

Giant stick insect

Grass stick insect

The wings of many butterflies and moths are not coloured the same above as below. Often the top colours are their ‘dress colours’, while the underside of the wings are camouflaged. So they can wave their showy flags while the coast is clear, but can disappear from view in an instant by snapping their wings shut.

This moth is camouflaged in mottled green-grey above and in blotchy yellow-brown from below. This means it can hide either in green foliage, lichen and bark, or in yellow leaf litter, as the need arises. Pretty clever don’t you think?

Diadem: the male diadem has a bold design that flashes with purple halos, and which he displays with abandon. When disturbed, this chap fled into a bush and folded his wings. Underneath, the wings look entirely different. The broad pale band breaks up the large wing shape into smaller brown areas with deceptive white spots. This could be quite effective inside a tangled bush. With some imagination, the wing edge resembles a fat caterpillar, sillouetted against the sky. A bird might just get taken in by that. In fact, this individual had already lost the tip of one wing, which bothered him not at all: he got away with its life.

High-contrast, ‘disruptive’ colouration, as in the example of the diadem, breaks up the outline of the animal, so it doesn’t stand out against the background. Bold patterns can give the insect false edges, obscuring its true shape or size, or simply confuse a predator so they don’t know where to strike.

Swallowtail: the markings on this swallowtail are potentially confusing in dappled forest lighting. The bright red eye spots, which the butterfly alternatively hides and exposes, may well have a scary effect.

The green and pale-yellow cross-banding on body and legs serve to hide the true size, shape and position of this eyed flower mantis. Flanges on legs and thorax add to the deception.

Deimatic (startling) colours and behaviour

Harmless insects prefer to go unnoticed, but when someone does spot them, then they may flash an (empty) warning, startling a predator momentarily. While the enemy reconsiders if an attack is advisable, even for a second, our friend can get away.

In mortal danger this hawk moth flashes bright red hind wings, which are normally kept hidden under camouflaged forewings. Some mantids, grasshoppers and stick insects also flash bright hindwings when attacked. Hawk moths are even able to hiss and squeak.

This tri-coloured tiger moth will flash its bright red abdomen when threatened, but cover it again to duck away. Tiger moths are even able to produce clicking sounds, through which they confuse bats’ sonar hunting habits.

When you bother a speckled emperor moth, which is perfectly camouflaged in leaf litter, it exposes two enormous circles that look for all the world like owl eyes. Pretty scary stuff!

Cape Autumn Widow: eye spots serve a double purpose: if the predator doesn’t get scared away, the eyes deflect the attack away from more vulnerable body parts. Eye spots are quite common. This Cape Autumn Widow has not just two but many small eye spots, dotted along the wing margin: little harmless target points that attract a predator’s attention to parts of the body the butterfly can do without. If one spot gets pecked out, there are more to go for the next round. It seems to work: eye spots are more often damaged than the rest of the wing.

By adding some ‘feelers’ to its eye spots, and wiggling its wings in a convincing way, the common hairtail butterfly is pretending that it is standing back-to-front. A bird is likely to peck at the wrong end. This decoy gives the butterfly at least one extra life.

In the atlas moth the rounded wing tips and wing margins resemble snakes, complete with eyes, nostrils, mouth, and scaly sides. Even the shading is correct. And with the see-through windows, where the background can shine through – whatever colour it may be – and all the rest of the confusing patterns, the entire effect is just too clever for words.

Hawk moth (top) and swallowtail (bottom) caterpillars also pretend to be snakes with eyes and even a forked tongue. These are serious scaring tactics! You have to be a very brave bird to see through such convincing disguises. (The red ‘tongue’ is an inflatable scent gland.) I once witnessed a Jack Russel dog attack a hawk moth caterpillar. The caterpillar’s jerks, squeaks and hisses drove the dog nearly insane. It gave a harmless caterpillar the same treatment as the puff adder it killed the day before.

Aposematic (warning) colouration

Many insects are of course dangerous, either because they have venomous barbs or stings, are poisonous or unpalatable to eat, produce toxic explosions, etc. Either way, they like to advertise this fact, warning predators to leave them alone. Bold, red, orange or yellow-and black markings shout “Danger!” in any language, a clear warning that every animal understands, and you see this pattern over and over in the insect world.

Poisonous insects often obtain their toxins from their food: plants that tried to ward them off with toxic chemicals. However, instead of getting poisoned, the insects store the chemicals in their body tissues, and thereby avoid getting eaten themselves.

By the way, poisons work by being ingested, venoms work by getting injected under the skin. ‘Toxin’ is an umbrella term that refers to any dangerous chemical produced by a living organism, including those that cause blisters or in any other way interfere with normal body function.

“I may be on the menu, but if you dare touch me you’ll break out in terrible blisters!” (Nairobifly, a rove beetle loaded with pederin – a most potent animal toxin – that may have been at the root of the third Biblical plague, see here.)

“I wouldn’t if I were you… I taste worse than I look. I’m a stinkbug.”

“Don’t touch! I’m a blister beetle… use your imagination what that means.”

“Stay away. I bite! In fact, us assassin bugs have the most painful insects bites of all!”

“Foul foul foul. You can see I’m not edible, can’t you?… Yeah, I’m loaded with cardiac glycosides – that’s heart poison in case you were wondering.” (Foam grasshoppers belong to the family Pyrgomorphidae, which means ‘garish forms’. To reinforce the message, they bubble foul-smelling and foul-tasting foam out of their thorax.)

Defensive mimicry

Insects who are not in the least dangerous themselves, may avoid getting eaten by looking like someone who is. This is called Batesian defensive mimicry. The mimicry often goes beyond colourations, and may involve body shape, behaviour, even sounds and smells. In Muellerian mimicry several unpalatable species share the same colouration to reinforce the message to would-be predators.

Monarch and mimic: a well-known example is the monarch butterfly (top). Its unforgettable colours and patterns warn birds to stay clear. It is poisonous. Birds that ignore the warning, soon vomit and learn their lesson the hard way.

Many other butterflies dress in similar colours (bottom image), whether they themselves are poisonous or not. This butterfly is not a monarch, though it may look like one.

Drone flies look a lot like bees, and they feed together with bees on the same flowers. But unlike bees they are totally harmless.

A wasp-mimicking soldier fly.

Here are three ant mimics: a young cricket, a spider, a young mantis. There are many more. Ants, with their fearsome stings and their predatory habits are the terror of the insect world. Harvester ants have the most venomous stings of them all.

Batesian mimicry is very sneaky, and it only works if there are not too many imposters around, which would confuse the message. That is why in some species, such as the diadem butterfly featured earlier, only the female commits the fraud (it is a mimic of the monarch butterfly) while the dispensable males stick to their specific colours. This gives their sisters a better fighting chance to continue the family line.

Aggressive camouflage/mimicry

Mantids use their camouflage not only to avoid being eaten (that too) but to avoid being spotted by their prey. When an ignoramus stumbles too close, they pounce. There are even some fascinating examples of aggressive mimicry:

On the left: mating cotton-stainer bugs. On the right: a cotton stainer assassin bug. It mimics the cotton strainers, presumably to go undetected, but then hunts and eats them. That’s plain nasty.

Again, on the top, the harmless model: a plant eating mealybug (source: D-Kuru/Wikimedia Commons). On the bottom: a deadly mimic, the carnivorous larva of a ladybird beetle. Here you see it eating a scale insect, but after this it may well go a-hunting, like a wolf in sheep’s clothing, among some real mealybugs. The disguise is aimed not so much at the (blind) mealybugs themselves, but at their ant ‘shepherds’. Ants eat the sweet honeydew produced by scale insects and mealybugs, and protect their ‘herds’ fiercely against other predatory insects. But with its clever disguise this ladybird larva gets past them.

An eyed flower mantis seems to be luring the bees closer with its spiral design. Somehow that bold pattern must have intrigued the bees, because they kept flying around the mantis, checking it out, trying to decide what it was.

Here is another bee species, coming too close for its own good.

I needn’t tell you… it didn’t take very long…

Two more functions of colour are worth mentioning:

These uncharacteristically white darkling beetles (toktokkies) live in the Namib desert. Their white wing covers, by reflecting the blazing sunlight, have a temperature regulating effect.

Bright, contrasting ‘uniforms’ may aid swarming grasshoppers to keep sight of each other. Desert locusts are normally dull brown and strictly solitary. But when food is abundant after heavy rains, and locust populations grow beyond a certain point, then the locusts suddenly change colour to bright yellow-and-black, become gregarious and start swarming.

You can meet some of the insects featured in this article in Marlies Craig’s newly published book, “What Insect are You? – Entomology for Everyone”. It is not yet available at book shops but you may order a copy from the author, or from Amazon. See www.whatinsectareyou.com for details.

About the author

Marlies Craig is an epidemiologist working in medical research. Though she originally studied Biology and Entomology, her love affair with insects is personal. In her newly published book, What Insect Are You? – Entomology for Everyone, she shares that passion with young and old. She hopes to kindle in children a deeper appreciation and understanding of nature, and show them why and how they can make a difference within their sphere of influence.

A Kloof Conservancy Project

About The Leopard’s Echo

The Leopard's Echo is Kloof Conservancy's bi-annual magazine. We feature articles on a variety of nature-related topics that celebrate our wonderful biodiversity.

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